I know off the top of my head Fictionwise accepts PayPal. Since you're getting a Libre which supports Adobe Digital Editions DRM, you won't need a credit card for those type of DRM'd ebooks. You just won't be able to use Secure eReader (PDB) format, which the Libre doesn't support anyway.
Of course, for DRM-free ebooks, the whole thing is a non-issue. Also, don't forget about the (legit) free eBook sites such as Project Gutenburg.